1. Field of the Invention
The present invention is directed to a formation of a microcontact printing stamp employed in the creation of microcircuitry where dimensional integrity and registration must be maintained at the micron level over very large distances that may be as long as half a meter. More particularly, the present invention is directed to a process of preparing a high precision microcontact printing stamp wherein monomeric shrinkage is compensated for by low temperature curing.
2. Background of the Prior Art
The process of microcontact printing, to create a very fine pitch pattern, is of recent vintage in the art of fabricating microcircuitry. This process is described in Kumar et al., Appl. Phys. Lett., 63 (14), 2002-2004 (October 1993) and Hidber et al., Langmuir, 12, 1375-1380 (1996). This process, which represents an alternative to photolithography in the fabrication of electronic components, especially where extremely fine line dimensions are required, requires the creation of a very fine pitch rubber stamp.
The very fine pitch rubber stamp utilized in microcontact printing is most often formed of an elastomeric material which is preferably silicone rubber. Those skilled in the art are aware that the term xe2x80x9csilicone rubberxe2x80x9d denotes polydimethylsiloxane (PDMS). In the current method of preparing rubber stamps used in high precision microcontact printing liquid, PDMS is introduced into a mold wherein a negative relief microcircuit pattern is expressed. The polymer is thereupon cured to produce a solidified rubber stamp which is removed from the mold. The solidified rubber stamp has a microcircuit pattern expressed in positive relief. It is this pattern that is transferred to a substrate in subsequent steps in the microcontact printing process.
The positive relief pattern provided on the rubber stamp is thereupon inked onto a substrate. Although there are several variations of microprinting methodology, commonly, the substrate is blanket coated with a thin gold film and the gold coated substrate is inked with an alkane thiol material transferred thereto by the stamp. Commonly, the alkane thiol material has the structural formula CH3xe2x80x94(CH2)18xe2x80x94CHSH2. It should, of course, be appreciated that other alkane thiol materials, as well as other inks, can be substituted for this alkane thiol.
Upon contact of the positive relief pattern of the stamp with the gold film, a monolayer of the ink, preferably an alkane thiol, having the desired microcircuit pattern, is transferred to the gold film layer. Alkane thiols form an ordered monolayer on gold by a self assembly process. Thus, a self assembled monolayer (SAM) of the desired pattern is formed on the gold layer. The SAM is tightly packed and well adhered to the gold. As such, the SAM acts as an etch resist upon the contact of a gold etching solution onto the stamped gold coated substrate.
In the next step, the inked substrate is immersed in a gold etching solution and all but the SAM is etched away to underlying layers below the gold layer. The SAM, which is invulnerable to the etch solution, is then stripped away leaving gold in the desired pattern.
The aforementioned description is set forth in the Kumar et al. technical article. The Hidber et al. technical article utilizes a different procedure wherein the aforementioned rubber stamp is inked with a palladium catalyst and a pattern is again stamped onto a substrate. The positive relief microcircuit pattern of palladium catalyst is subsequently immersed in an electroless plating solution which induces the desired microcircuit pattern by electroless plating.
The aforementioned description makes it apparent that faithful reproduction of the microcircuit pattern of the printing stamp onto the substrate is critical, especially when the pattern is of both fine pitch and of very large overall dimensions. For example, if microcontact printing is used to produce microcircuitry on flat panel displays, it may require 5 micron sized features to be accurately registered to one another within 1 micron across a linear distance of 15 inches.
In turn, faithful reproduction of the microcircuit onto the substrate requires the fabrication of a microcontact printing stamp that faithfully reproduces the desired microcircuit. This challenge to produce a high precision microcircuit printing stamp is magnified by the additional requirement that this formation of a microcontact printing stamp be simple and cost effective. This latter requirement is emphasized because a primary application of this technology is the manufacture of flat panel displays. Flat panel displays must be produced at low cost and yet must meet the stringent tolerance criteria mentioned above.
In the past microcontact printing could not meet this challenge. This was because microcontact printing stamps could not satisfy the registration requirement because of shrinkage during their preparation. That is, the elastomeric polymer would shrink during its curing in the mold. As those skilled in the art are aware, when an elastomeric polymer, such as silicone rubber, cures in a mold it shrinks in an amount of between about 0.1% to about 4%.
Thus, it is apparent that there is a strong need in the art for a new microcontact printing stamp forming process that provides a stamp that provides good registration by compensating for the shrinkage that occurs during curing in the mold.
A new process has now been developed which results in the formation of a high precision microcontact printing stamp providing a positive relief microcircuit pattern faithful to the dimensional requirements of the desired microcircuit.
In accordance with the present invention a process of fabricating a high precision microcontact printing stamp is provided. In this process an elastomeric monomer or oligomer is introduced into a mold housing in which a photoresist master, defining a microcircuit in negative relief, disposed on a substrate, is situated above a stamp backplane. The mold housing containing the elastomeric monomer or oligomer is allowed to cure at ambient temperature for a period of about 4 days to about 1 week. A molded product is removed from the mold housing and the photoresist master is peeled away to provide a high precision microcontact printing stamp expressing the microcircuit in positive relief having significantly lower distortion than heretofore available.
In further accordance with the present invention a second embodiment of the process of fabricating a microcontact printing stamp, identical to the first embodiment but for the temperature at which curing occurs, is provided. Whereas curing in the first embodiment takes place at ambient temperature, curing in the second embodiment occurs at a temperature between about 1xc2x0 C. and about 5xc2x0 C. below ambient temperature for a period of between about 5 days and about 1 week. This process produces a high precision microcontact printing stamp having almost no detectable distortion.
Yet a third embodiment of the process of the present invention is one in which the process of the second embodiment is repeated but for the further requirement that the elastomeric polymer be photocurable. In this process the photocurable elastomeric polymer is exposed to the same temperature utilized in the second embodiment but, in addition, the polymer in the mold is exposed to ultraviolet light. The microcontact printing stamp product of the process of the third embodiment is substantially identical to that of the second embodiment but for the rate of curing. Whereas the second embodiment cures in a period of between about 5 days and about 1 week, the process of the third embodiment cures within a period of between about 30 seconds and about 1 hour.